What is meant by pervasive developmental disorder in young people?

PDD: Pervasive developmental disorder is a label/diagnosis given to those with some but not all of the criteria for autistic spectrum disorder. It reflects a less severe disorder. It is used for significant impairment in development of reciprocal social interaction or communication skills or when stereotyped behaviors, interests & activities are present.

Slow: This is a modern term for what used to be called "slow". Kids or adults who had no definite diagnosis for their poor development in all areas of function are lumped into this category.